MIT researchers have developed a way to manufacture and integrate artificial atoms with photonic circuitry to produce a quantum chip that could be used for large-scale quantum systems. Using a hybrid, modular approach, the researchers built what they believe to be the largest integrated artificial-atom photonics chip yet. Instead of trying to build a large quantum chip entirely in diamond, the researchers integrated quantum microchiplets, or QMCs — diamond waveguide arrays containing highly coherent color centers — on a photonic integrated circuit (PIC). The artificial atoms in the chiplets consist of the diamond color centers. Each center functions as an atom-like emitter whose spin states can form a qubit. When prodded with visible light and microwaves, the artificial atoms emit photons that carry the quantum information. The researchers used semiconductor fabrication techniques to make the chiplets, then selected the highest quality qubit modules from the chiplets they produced. The microchiplets were placed on a PIC, which provided the underlying architecture to route and switch photons between qubit modules with low loss. The PIC platform was made of aluminum nitride. Using this approach, the researchers were able to connect 128 qubits on one platform. They said that the qubits are stable and long-lived, and their emissions can be tuned within the circuit to produce spectrally indistinguishable photons. This graphic depicts a stylized rendering of the quantum photonic chip and its assembly process. The bottom half of the image shows a functioning quantum microchiplet (QMC), which emits single-photon pulses that are routed and manipulated on a photonic integrated circuit (PIC). The top half of the image shows how this chip is made: Diamond QMCs are fabricated separately and then transferred into the PIC. Courtesy of Noel H. Wan. Diamond color centers have emerged as leading solid-state qubits because they enable on-demand remote entanglement, coherent control, and memory-enhanced quantum communication. However, according to researcher Noel Wan, “the bottleneck with this platform is actually building a system and device architecture that can scale to thousands and millions of qubits. “Artificial atoms are in a solid crystal, and unwanted contamination can affect important quantum properties such as coherence times,” Wan said. “Furthermore, variations within the crystal can cause the qubits to be different from one another, and that makes it difficult to scale these systems.” While the MIT hybrid platform could offer a scalable process to produce artificial-atom photonics chips, the next step will be to test its processing skills. “This is a proof of concept that solid-state qubit emitters are very scalable quantum technologies,” Wan said. “In order to process quantum information, the next step would be to control these large numbers of qubits and also induce interactions between them.” The ability to assemble large numbers of nearly indistinguishable and tunable artificial atoms into phase-stable PICs is a key step toward multiplexed quantum repeaters and general-purpose quantum processors. “In the past 20 years of quantum engineering, it has been the ultimate vision to manufacture such artificial qubit systems at volumes comparable to integrated electronics,” professor Dirk Englund said. “Although there has been remarkable progress in this very active area of research, fabrication and materials complications have thus far yielded just two to three emitters per photonic system.” Other types of diamond color centers could be used for the qubits in the MIT team’s chip design, the researchers said. “Because the integration technique is hybrid and modular, we can choose the best material suitable for each component, rather than relying on natural properties of only one material, thus allowing us to combine the best properties of each disparate material into one system,” researcher Tsung-Ju Lu said. The research was published in Nature (www.doi.org/10.1038/s41586-020-2441-3).